Laminated polyester film

ABSTRACT

A laminated polyester film in which a coating layer whose main constituent component is a water-soluble or water-dispersible resin is provided on at least one side of a polyester film, wherein the water resistance value of the film is at least 90%, the discoloration after melt molding is no more than 10, and the haze value change after heating is no more than 20%. Adhesion, water resistance, recoverability, and whitening resistance after heating are all excellent.

FIELD OF THE INVENTION

This invention relates to a laminated polyester film having a coatinglayer. More particularly, the present invention relates to a laminatedpolyester film having a coating layer, which has excellent adhesion witha wide range of materials, such as photographic photosensitive layers,diazo photosensitive layers, mat layers, magnetic layers, ink layers,adhesive agent layers, thermosetting resin layers, UV setting resinlayers, and vapor deposited layers of metal or inorganic oxides, whichhas good water resistance and resistance to whitening due to heating,and which, when recovered in the form of scrap film, can be reused as afilm raw material.

BACKGROUND OF THE INVENTION

Laminated polyester films which are adhesive find use in a wide range ofapplications, such as magnetic tape base films, insulating tapes,photographic films, liquid crystal components, antireflective films,tracing films, and food packaging films. Because a polyester film itselfdoes not have sufficient adhesion, its adhesion is generally improved byproviding an anchor coat layer to the polyester film.

Many different resins have been proposed as anchor coating resins up tonow. For instance, the use of a water-soluble or water-dispersiblepolyester resin or acrylic resin for a film with relatively highpolarity, typically a polyester film, has been proposed in JapaneseUnexamined Patent Publication S54-43017, Japanese Examined PatentPublication S49-10243, Japanese Unexamined Patent Publications S52-19787and S58-124651, and elsewhere. The effect of these prior art techniques,however, was inadequate in terms of improving adhesion.

In order to improve the adhesion of a polyester film, it has beenproposed to use various modified polyester resins (mainly involvinggraft modification) as an anchor coating resin in Japanese UnexaminedPatent Publications H2-3307, H2-171243, H2-310048, and H3-273015,Japanese Examined Patent Publication H3-67626, and elsewhere.Nevertheless, while adhesion is indeed increased by using the graftmodified polyester resin as an anchor coating resin, a problem is thatthe adhesion is poor under wet conditions.

Accordingly, it has been proposed to increase adhesion under wetconditions by adding a crosslinking agent in Japanese Examined PatentPublications H5-744633, H6-24765, H6-39154, and H6-39548 and elsewhere.

The use of such crosslinking agents improves adhesion under wetconditions. Nevertheless, during the manufacture of a polyester filmwhen the film which does not become a finished product, i.e., scrapfilm, is melted and molded into pellets and reused as a film rawmaterial, the resulting film is so low in quality as to make such reuseimpractical. Therefore, even though such a coated polyester film hasexcellent water resistance and adhesion, the scrap film that does notbecome a finished product during film manufacture is discarded, or usedfor limited applications or added in a small amount. This increasesproduction cost and also poses a problem from the standpoint of theenvironmental load imposed by the scrap film.

Other problems are that under the high-temperature environmentencountered during or after processing to the film in opticalapplications and so forth, the film whitens and loses transparency, ormicroscopic bumps form on its surface. This film whitening and theformation of microscopic surface bumps occur when crystals of polyesteroligomer in the film precipitate at the surface.

As a method for suppressing oligomer precipitation at the film surface,it has been proposed to use a polyester with a low oligomer contentmanufactured by solid phase polymerization (Japanese Unexamined PatentPublications S55-89330 and S55-189331), or to cover a film surface witha polyester having a low oligomer content (Japanese Unexamined PatentPublication H11-300918), etc. However, adhesion, water resistance andrecoverability cannot all be improved by these methods alone, and evenif these methods are combined with a known coating film technique, it isstill impossible to obtain a laminated polyester film whose adhesion,water resistance, recoverability, and resistance to whitening due toheating are all excellent.

Thus, it is an object of the present invention to solve theabove-mentioned problems encountered with prior art, and provide alaminated polyester film whose adhesion, water resistance,recoverability, and resistance to whitening due to heating are allexcellent.

SUMMARY OF THE INVENTION

The present invention was accomplished in light of the above situation,and the laminated polyester film with which the stated object can beachieved is as follows.

The first invention of the present invention is a laminated polyesterfilm prepared by forming a coating layer comprising as main constituentcomponent a water-soluble or water-dispersible resin on at least oneside of a polyester film, the laminated polyester film having a waterresistance value of at least 90%, a discoloration value after meltmolding of not more than 10, and a haze value change after heating ofnot more than 20%.

The second invention is the laminated polyester film according to thefirst invention, wherein the water-soluble or water-dispersible resin isone or more resins or a copolymer resin comprising two or more resinsselected from the group consisting of aqueous acrylic resins with anacid value of at least 200 eq/t and aqueous aromatic polyester resins.

The third invention is the laminated polyester film according to thesecond invention, wherein the water-soluble or water-dispersible resincontains at least 5 wt % of a radical polymer of at least one monomercomprising an acid anhydride containing a double bond.

The fourth invention is the laminated polyester film according to thefirst, second, or third invention, wherein the coating weight of thecoating layer after drying is 0.01 to 1.0 g/m².

The fifth invention is the laminated polyester film according to thefirst invention, wherein the film is used for printing applications.

The sixth invention is the laminated polyester film according to thefirst invention, wherein the film is used as a substrate film for anoptical component.

Function

The laminated polyester film of the present invention needs to have awater resistance value at the coating layer surface of at least 90%,with 95% or higher being preferable. If the water resistance value isless than 90%, the adhesion under wet conditions would be inadequatewhen an ink layer is formed on the coating layer surface of thelaminated polyester film. The water resistance value is defined asfollows in the present invention. The coating layer surface of thelaminated film is coated with a UV-cured sealed ink, and after UVcuring, a boiling treatment under pressure is performed for 1 hour at120° C., and then the peel test according to JIS K 5400 is conducted.The water resistance value defined as the percent area (%) of theremaining ink which is not peeled off.

The laminated polyester film of the present invention needs to have adiscoloration value after melt molding of not more than 10. If thediscoloration value is over 10, there will be a marked drop in filmquality when recovered pellets are used as a film raw material.

The “discoloration value after melt molding” as defined in the presentinvention is a parameter expressed by the difference in the color bvalue between a pellet obtained by the melt molding of a laminatedpolyester film, and the laminated polyester film prior to this meltmolding.

More specifically, the laminated polyester film is cut into strips anddried under reduced pressure, and the strips are melt extruded at atemperature of 280° C. with a model test apparatus, and the extrudate iscooled in water and then cut into pellets. These pellets willhereinafter be referred to as “recovered pellets”. The color b values ofthe recovered pellets and of the laminated polyester film prior to thetest are measured, and the difference between the two values is definedas the discoloration after melt molding. The “color b value” refers tothe b value measured in Lab space with a photoelectric color meter.

The background in which this parameter is used will be described below.

When a polyester film is manufactured, scrap film which is not used inthe finished product is invariably generated at the ends held by clipsduring transverse stretching with a tenter, at the ends which do notmeet the finished product width requirement during slitting, inbelow-grade rolled films, at the beginning and the end of filmmanufacture, when conditions are changed, and when malfunctions occur.In general, the above-mentioned scrap film is broken into flakes andthen melted in an extruder, and the extrudate is discharged from a diein the form of a strand into water and then cut into pellets. Theobtained recovered pellets are re-molded and reused as a film rawmaterial. However, because of heat hysteresis built up during themanufacture of the recovered pellets, a film produced using recoveredpellets made from a laminated polyester film having a coating layer endsup containing foreign matter which results in fish-eyes, or ends uphaving low quality due to coloration and so forth. Consequently, scrapfilm not used as a finished product cannot be reused as a film rawmaterial in applications in which transparency or defects such as coarseprotrusions or fish-eyes caused by the foreign matter can pose aproblem, particularly in optical applications.

We analyzed this phenomenon in detail, and found in a model test thatwhen a laminated polyester film having a coating layer is re-melted andre-molded into pellets and biaxial stretching is carried out using thesepellets as a film raw material to thereby give a film, a discolorationvalue (discoloration value after melt molding) determined with respectto the obtained film can be used as a parameter for estimating thequality of a laminated film which is actually manufactured using a filmraw material containing recovered pellets. As a result, in order tominimize the decrease in the quality of a film in which recoveredpellets are used, that is, in order to obtain a recoverable laminatedpolyester film, the discoloration after melt molding must be 10 or less.

Furthermore, the laminated polyester film of the present invention needsto have a haze value change after heating of not more than 20%, with 15%or less being preferred, and 10% or less being particularly preferable.If the change in haze value is over 20%, whitening of the film becomesso pronounced that the effect on appearance and performance cannot beignored, particularly in applications in which a heat treatment isperformed in an after-processing step, such as in printing or opticalapplications. “Haze value change after heating” refers to the differencebetween the haze value of the laminated polyester film after heating itfor 30 minutes at 150° C., and the haze value of the laminated polyesterfilm prior to the heating. When the laminated polyester film of thepresent invention is to be heat-treated in an after-processing step,this parameter is used in quality control to estimate, at the productionstage of the laminated polyester film, the degree of whitening of thefilm caused by the heat-treatment.

Embodiments of the laminated polyester film of the present inventionwill be described below in detail.

□Polyester Resin Used for Substrate Film□

The polyester resin used as the substrate film in the present inventionis polyethylene terephthalate, polybutylene terephthalate,polyethylene-2,6-naphthalate, or a copolymer whose main components arethe structural components of these resins. Of these, a biaxiallyoriented film formed from polyethylene terephthalate is particularlypreferable. There are no particular restrictions on the method formanufacturing these polymers, and solid phase polymerization may beperformed, or a polyester whose oligomer content has been reduced bysubjecting these polymers to solvent extraction or the like may be used.

A polycondensation catalyst (a transesterification catalyst may also beused in the case of transesterification) and a thermal stabilizer suchas a phosphoric acid or a phosphoric acid compound are used as essentialcomponents in the polyester film used in the present invention.Furthermore, addition of a suitable amount of an alkali metal salt or analkaline earth metal salt is preferable since a sheet of moltenpolyester resin can be solidified on a rotating cooling roll byelectropinning, to thereby obtain an unstretched sheet of uniformthickness. It is also preferable to add inert particles to the substratepolyester film in order to improve the handling characteristics of thefilm, such as slidability, windability and blocking resistance, and itswear characteristics such as wear resistance and scratch resistance. Ifdesired, various other additives can also be added to the polyesterresin. Examples of such additives include antioxidants, light-resistingagents, anti-gelling agents, organic lubricants, antistatic agents, UVabsorbers and surfactants.

When the laminated polyester film of the present invention is used as abase film for an optical component, it needs to have excellent handlingcharacteristics while retaining a high degree of transparency□Therefore, it is preferable to add fine particles only in the coatinglayer, with substantially no particles being contained in the substratefilm.

Coating Solution

In the present invention, there are no particular restrictions on thewater-soluble or water-dispersible resin used for the coating layer onthe substrate film, but examples include aqueous polyester resins,aqueous acrylic resins, and aqueous polyurethane resins. Of these, it ispreferable to use one or more resins selected from the group consistingof aqueous acrylic resins with an acid value of at least 200 eq/t andaqueous aromatic polyester resins, or a copolymer of two or more ofthese. The term copolymer here includes both block and graft copolymers.

“Aromatic polyester resin” refers to a polyester resin in which at least30 mol % of the acid component is an aromatic dicarboxylic acidcomponent. If the aromatic dicarboxylic acid component accounts for lessthan 30 mol %, the polyester resin becomes much more prone tohydrolysis, and water resistance is lowered.

The acid value of the above-mentioned acrylic resin is determined byusing an ethanolic potassium hydroxide solution of known concentrationto titrate the solids obtained after a resin solution or the like hasbeen dried for 2 hours at 80° C. under a reduced pressure of 100 Pa. Ifthe acid value is less than 200 eq/t, the acrylic resin is notsufficiently water-soluble or water-dispersible. The acrylic resin, whenhaving an acid value of at least 200 eq/t, is water-soluble orwater-dispersible. Polar groups must be contained in the molecules inorder for the acid value to be at least 200 eq/t. However, stable polargroups which remain unchanged even when heated, as is the case withsodium sulfonate, are actually undesirable because they lower the waterresistance of the coating layer.

Examples of polar groups which do not adversely affect the waterresistance of the coating layer are carboxylic acid amine salts which,when heated, decompose to thereby possess decreased polarity. The aminewhich is used needs to vaporize under the coating film dryingconditions, and examples thereof include ammonium, diethylamine,triethylamine and the like.

It is more preferable that one or more resins selected from the groupconsisting of aqueous acrylic resins with an acid value of at least 200eq/t or aqueous aromatic polyester resins, or a copolymer of two or moreof these, contains at least 5 wt % of a radical polymer of at least onemonomer comprising an acid anhydride containing a double bond. The waterresistance effect would not be fully produced when the radical polymercontent is less than 5 wt %.

Introduction of the above-mentioned acid anhydride into the resinenables the resin molecules to be crosslinked. Specifically, throughhydrolysis or the like in the coating solution, the acid anhydride inthe resin molecule changes into a carboxylic acid which intermolecularlyreacts, due to heat hysteresis during drying and film manufacture, withan acid anhydride or active hydrogen group of other molecules to producean ester group or the like, crosslinking the resin and thereby impartingwater resistance, resistance to whitening due to heating, and otherproperties.

Examples of monomers containing an double bond-containing acid anhydrideinclude maleic anhydride, itaconic anhydride, 2,5-norbornenedicarboxylicanhydride, and tetrahydrophthalic anhydride. The radical polymer mayalso be a copolymer of such monomer and other polymerizable unsaturatedmonomer(s).

Examples of said other polymerizable unsaturated monomers include (1)fumaric acid, monoethyl fumarate, diethyl fumarate, dibutyl fumarate andother monoesters and diesters of fumaric acid, (2) maleic acid,monoethyl maleate, diethyl maleate, dibutyl maleate and other monoestersand diesters of maleic acid, (3) itaconic acid, monoesters and diestersof itaconic acid, (4) phenylmaleimide and other maleimides, (5) styrene,α-methylstyrene, t-butylstyrene, chloromethylstyrene and other styrenederivatives, (6) vinyltoluene, divinylbenzene and so forth, (7) alkylacrylates, alkyl methacrylates (where the alkyl group may be methylgroup, ethyl group, n-propyl group, isopropyl group, n-butyl group,isobutyl group, t-butyl group, 2-ethylhexyl group, cyclohexyl group,phenyl group, benzyl group, phenylethyl group, or the like), and otheracrylic polymerizable monomers, (8) 2-hydroxyethyl acrylate,2-hydroxyethyl methacrylate, 2-hydroxypropyl acrylate, 2-hydroxypropylmethacrylate, and other hydroxyl-containing acrylic monomers, (9)acrylamide, methacrylamide, N-methylmethacrylamide, N-methylacrylamide,N-methylolacrylamide, N-methylolmethacrylamide,N,N-dimethylolacrylamide, N-methoxymethylacrylamide,N-methoxymethylmethacrylamide, N-phenylacrylamide, and other amidegroup-containing acrylic monomers, (10) N,N-diethylaminoethyl acrylate,N,N-diethylaminoethyl methacrylate, and other amino group-containingacrylic monomers, (11) glycidyl acrylate, glycidyl methacrylate, andother epoxy group-containing acrylic monomers, and (12) acrylic acid,methacrylic acid, salts thereof (sodium, potassium, and ammonium salts)and other acrylic monomers containing carboxyl group or a salt thereof.

The main constituent components of the coating solution used to form thecoating layer of the present invention are a water-soluble orwater-dispersible resin and an aqueous solvent. As discussed above, itis even more preferable that the water-soluble or water-dispersibleresin is one or more resins selected from the group consisting ofaqueous acrylic resins with an acid value of at least 200 eq/t andaqueous aromatic polyester resins, or a copolymer of two or more ofthese, containing at least 5 wt % of a radical polymer of at least onemonomer comprising a double bond-containing acid anhydride.

In this case, addition of an acid compound during the preparation of thecoating solution is particularly preferable. This addition of an acidcompound increases the crosslinking of the resin by promoting acidanhydride conversion and esterification reactions of the carboxylic acidgroups in the resin, and is therefore favorable in terms of achieving awater resistance value of at least 90%, which is an importantrequirement in the laminated polyester film of the present invention.

It is preferable that the acid compound is added in an amount of 1 to 10wt % based on the resin. A variety of compounds can be used as the acidcompound, but preferable is a low-boiling carboxylic acid which readilyvaporizes by the heat during film manufacture, seldom remains in thecoating layer and less adversely affects the film when it remains.Examples of low-boiling carboxylic acids include formic acid, aceticacid, propionic acid, butyric acid, valeric acid, heptanoic acid and thelike.

In the present invention, in order to satisfy the desired waterresistance and discoloration values, it is preferable to use a nitrogenatom-free or phenol-free crosslinking agent in the crosslinking reactionusing the above-mentioned acid anhydride.

A nitrogen atom-containing or phenol-containing crosslinking agent isoxidized and decomposed when subjected to heat or the like and forms acompound having conjugated structure centered around the nitrogen atomand aromatic ring. As a result, pronounced coloration occurs.

In the present invention, however, the use of these crosslinking agentsis not absolutely ruled out, and as long as the water resistance anddiscoloration values of the present invention are satisfied, thesecrosslinking agents can be used in an appropriate amount according tothe type of the crosslinking agent (curing resin).

Examples of the nitrogen-containing crosslinking agent include (1)adducts of formaldehyde with urea, melamine, benzoguanamine or the like,(2) amino resins such as alkyl ether compounds composed of these adductsand an alcohol having 1 to 6 carbon atoms, (3) polyfunctional epoxycompounds, (4) polyfunctional isocyanate compounds, (5) blockedisocyanate compounds, (6) polyfunctional aziridine compounds, and (7)oxazoline compounds.

Examples of the amino resins mentioned in (2) above include methoxylatedmethylolurea, methoxylated methylol N,N-ethyleneurea, methoxylatedmethyloldicyandiamide, methoxylated methylolmelamine, methoxylatedmethylolbenzoguanamine, butoxylated methylolmelamine, butoxylatedmethylolbenzoguanamine or the like. Of these, methoxylatedmethylolmelamine, butoxylated methylolmelamine, methylolatedbenzoguanamine are preferable.

Examples of the polyfunctional epoxy compounds mentioned in (3) aboveinclude a diglycidyl ether of bisphenol A and oligomers thereof,diglycidyl ether of hydrogenated bisphenol A and oligomers thereof,orthophthalic acid diglycidyl ester, isophthalic acid diglycidyl ester,terephthalic acid diglycidyl ester, p-hydroxybenzoic acid diglycidylester, tetrahydrophthalic acid diglycidyl ester, hexahydrophthalic aciddiglycidyl ester, succinic acid diglycidyl ester, adipic acid diglycidylester, sebacic acid diglycidyl ester, ethylene glycol diglycidyl ether,propylene glycol diglycidyl ether, 1,4-butanediol diglycidyl ether,1,6-hexanediol diglycidyl ether, polyalkylene glycol diglycidyl ethers,trimellitic acid triglycidyl ester, triglycidyl isocyanurate,1,4-diglycidyloxybenzene, diglycidylpropyleneurea, glycerol triglycidylether, trimethylolpropane triglycidyl ether, pentaerythritol triglycidylether, and triglycidyl ether of a glycerol alkylene oxide adduct.

Examples of the polyfunctional isocyanate compounds mentioned in (4)above include low- or high-molecular weight aromatic or aliphaticdiisocyanates, and trivalent or higher polyisocyanates.

Examples of polyisocyanates include tetramethylene diisocyanate,hexamethylene diisocyanate, toluene diisocyanate, diphenylmethanediisocyanate, hydrogenated diphenylmethane diisocyanate, xylylenediisocyanate, hydrogenated xylylene diisocyanate, isophoronediisocyanate, and trimers of these isocyanate compounds.

Moreover, compounds having one or more terminal isocyanate groups may beused. They are obtained by reacting an excess amount of one of theseisocyanate compounds with a low-molecular weight active hydrogencompound such as ethylene glycol, propylene glycol, trimethylolpropane,glycerol, sorbitol, ethylenediamine, monoethanolamine, diethanolamine ortriethanolamine, or with a high-molecular weight active hydrogencompound such as a polyester polyol, polyether polyol or polyamide.

The blocked isocyanate mentioned in (5) above can be synthesized by anaddition reaction of one of the above-mentioned isocyanate compoundswith a blocking agent by a known method.

Examples of the isocyanate blocking agents include (a) phenol compoundssuch as phenol, cresol, xylenol, resorcinol, nitrophenol, andchlorophenol, (b) thiophenol compounds such as thiophenol andmethylthiophenol, (c) oximes such as acetoxime, methyl ethyl ketoxime,and cyclohexanone oxime, (d) alcohols such as methanol, ethanol,propanol and butanol, (e) halogen substituted alcohols such asethylenechlorohydrine and 1,3-dichloro-2-propanol, (f) tertiary alcoholssuch as t-butanol and t-pentanol, (g) lactams such as ε-caprolactam,δ-valerolactam, ν-butyrolactam, and β-propyllactam, (h) aromatic amines,(i) imides, (j) active methylene compounds such as acetylacetone,acetoacetate and ethyl malonate, (k) mercaptans, (l) imines, (m) ureas,(n) diaryl compounds, and (o) sodium disulfite.

Examples of phenol-containing crosslinking agents include phenolformaldehyde resins which are condensates of formaldehyde and alkylatedphenols, cresols and so forth.

Examples of phenol formaldehyde resins include condensates offormaldehyde and one or more phenols such as alkylated (methyl, ethyl,propyl, isopropyl, or butyl) phenol, p-tert-amylphenol,4,4′-sec-butylidenephenol, p-tert-butylphenol, o-cresol, m-cresol,p-cresol, p-cyclohexylphenol, 4,4′-isopropylidenephenol, p-nonylphenol,p-octylphenol, 3-pentadecylphenol, phenol, phenyl-o-cresol,p-phenylphenol and xylenol.

In the present invention, it is preferable that the coating solutionused to form the coating layer is an aqueous coating solution. Inapplying this aqueous coating solution to the substrate film surface, itis preferable to add a suitable amount of a known anionic or nonionicsurfactant in order to improve the wettability of the substrate film andto apply the coating solution uniformly.

In order to impart other functionalities to the laminated film such ashandling characteristics, antistatic properties or antibacterialproperties, the aqueous coating solution may contain additives such asinorganic and/or heat-resistant polymer particles, an antistatic agent,a UV absorber, an organic lubricant, an antibacterial agent or aphoto-oxidizing agent.

Especially, when the substrate polyester film contains substantially noinert particles, it is preferable, in order to improve the handlingcharacteristics of the film, that inorganic and/or heat-resistantpolymer particles be added to the aqueous coating solution so as to formbumps on the coating layer surface. Furthermore, because the coatingsolution is aqueous, a plurality of water-soluble resins,water-dispersible resins, emulsions, or the like may be added to thecoating solution in order to improve performance.

In addition to water, an alcohol such as ethanol, isopropyl alcohol orbenzyl alcohol may be added as a solvent to the coating solution in anamount of less than 50 wt % based on the total coating solution.Furthermore, an organic solvent other than the alcohol may also be addedin an amount effective for dissolution, provided that the amount thereofis less than 10 wt %. It is preferable, however, that the combinedamount of the alcohol and said other organic solvent in the coatingsolution is less than 50 wt %.

Such organic solvents, when present in an amount of less than 50 wt %,produce the effect of allowing the applied coating solution to dryfaster and the effect of giving a coating layer having improvedappearance compared to the case where water alone is used. The use oforganic solvents in an amount of 50 wt % or higher increases theevaporation rate of the solvents and causes a change in theconcentration of the coating solution while it is being applied, withthe result that the viscosity of the coating solution increases and thesolution will be more difficult to apply. As a result, the resultingcoating layer is likely to have an inferior appearance. Furthermore,exceeding this amount is undesirable in terms of the environment,operators' health, fire hazard and so on.

Manufacturing a Laminated Polyester Film

The method for manufacturing the laminated polyester film of the presentinvention will now be described by using a polyethylene terephthalate(hereinafter referred to as PET) film as an example, but naturally thepresent invention is not limited to this embodiment.

A PET resin is thoroughly vacuum dried and fed to an extruder. Themolten PET resin at approximately 280° C. is melt extruded in the formof a sheet from a T-die onto a rotating cooling roll, and is cooled andsolidified by electropinning to obtain an unstretched PET sheet. Thisunstretched PET sheet may be in a single layer structure, or in amultilayer structure obtained by co-extrusion. For applications whichdemand a high level of transparency, such as optical applications, it ispreferable that the PET resin contain substantially no inert particles.

The unstretched PET sheet thus obtained is stretched 2.5 to 5.0 times inthe longitudinal with a roll heated at 80 to 120° C., to give amonoaxially oriented PET film. The ends of the film are clamped withclips, and the film is guided into a hot-air zone heated at 70 to 140°C. and stretched 2.5 to 5.0 times in the transverse direction. It isthen conveyed into a heat treatment zone of 160 to 240° C., where it isheat treated for 1 to 60 seconds to complete crystal orientation.

At some step in the course of this film manufacture, at least one sideof the PET film is coated with an aqueous coating solution comprising asmain components the above-mentioned water-soluble or water-dispersibleresin and an aqueous solvent. The coating layer may be formed on onlyone side of the PET film, but forming a coating layer on both sides ofthe PET film is even more effective for preventing the oligomerprecipitation at the film surface and for minimizing the increase inhaze value after film heating.

It is preferable that the solids concentration of the resin compositionin the aqueous coating solution is 5 to 35 wt %, more preferably 7 to 15wt %.

Any known methods can be employed to coat the PET film with this aqueouscoating solution. Examples include reverse roll coating, gravurecoating, kiss coating, die coating, roll brushing, spray coating, airknife coating, wire bar coating, pipe doctoring, dip coating, andcurtain coating. These methods can be performed singly or incombinations.

The coating layer may be provided by coating a biaxially oriented PETfilm with the above-mentioned coating solution and then drying thecoating (offline coating), or by coating an unstretched or monoaxiallyoriented PET film with the above-mentioned coating solution, drying thecoating, and then drawing at least in one direction and performing athermosetting treatment (inline coating).

The latter method (inline coating) is preferred from the standpoint ofthe effect of the present invention. After being coated with the coatingsolution, the film is guided to a tenter and heated for transversedrawing and thermosetting. A stable coating layer can be formed herebecause of the thermal crosslinking reaction.

In the case of the so-called inline coating method, in which anunstretched or uniaxially oriented PET film is coated with theabove-mentioned coating solution and then dried and stretched, thetemperature and time is selected such that only the water or othersolvent is removed in the drying step following coating and that thecoating layer does not undergo a crosslinking reaction.

It is preferable that the drying temperature is 70 to 140° C., and whilethe drying time is adjusted as dictated by the coating solution and thecoating amount, it is preferable that the product of multiplying thetemperature (° C.) by the time (seconds) is 3000 or less. If thisproduct is over 3000, a crosslinking reaction will commence in thecoating layer prior to stretching, causing cracking and so forth in thecoating layer, thereby making it difficult to achieve the object of thepresent invention.

The stretched film is usually subjected to a relaxation treatment ofabout 2 to 10%, and in the present invention the coating layer ispreferably heated with an infrared heater in a low-strain state, thatis, a state in which the length of the film is fixed in the transversedirection. It is particularly preferable that the above-mentionedcoating layer be heated for a short time of 0.5 to 1 second at 250 to260° C.

The use of this method further promotes the crosslinking of the resin inthe coating layer, and the resin becomes even stronger, thereby betterachieving the effects of the present invention, i.e., water resistanceand resistance to whitening due to heating.

It is not preferable that either the temperature or the time for theheating with the infrared heater is over the preferable range givenabove, since the film will be prone to crystallization or melting. Onthe other hand, if the heating temperature or heating time is below thepreferable range given above, inadequate crosslinking of the coatinglayer tends to occur, and results in insufficient water resistance andresistance to whitening due to heating.

It is preferable that the coating amount of the coating layer in thelaminated polyester film eventually obtained in the present invention isfrom 0.01 to 1.0 g/m². A coating amount of less than 0.01 g/m² gives acoating layer having substantially no adhesion. On the other hand, orcoating amount exceeding 1.0 g/m² will adversely affect recoverability.

The coating layer of the laminated polyester film of the presentinvention has good adhesion to a variety of materials, but in order tofurther improve adhesion and printability, the coating layer may befurther subjected to surface treatment by corona discharge treatment,flame treatment, electron beam irradiation or the like, if so desired.

The coating layer of the laminated polyester film obtained according tothe present invention affords good adhesive strength in a wide range ofapplications. Specific examples include photographic photosensitivelayers, diazo photosensitive layers, mat layers, magnetic layers, inkjet ink receiving layers, hard coat layers, UV-cured resins,thermosetting resins, printing inks, UV inks, adhesive agents for drylamination or extrusion lamination, thin film layers obtained by plasmapolymerization, CVD, ion plating, sputtering, electron beam deposition,or vacuum vapor deposition of metals or inorganic substances or oxidesof these, and organic barrier layers.

EXAMPLES

The present invention will be described below in detail with referenceto examples and comparative examples. However, the present invention isnot, of course, limited to the examples given below. The evaluationmethods used in the present invention are as follows.

(1) Adhesion

The coating layer side of a film was coated with a hard coating agent(Seikabeam EFX01 (B), made by Dainichiseika) using a #8 wire bar. Thecoating was dried for 1 minute at 70° C. to remove the solvent. Then, ahigh pressure mercury vapor lamp was used to form a hard coating layerwith a thickness of 3 μm at an irradiation energy of 200 mJ/cm² and anirradiation distance of 15 cm while the film was fed at a speed of 5m/minute.

The film thus obtained was tested for adhesion according to the testmethod described in JIS K 5400, section 8.5.1. Specifically, using acutter guide with a spacing of 2 mm, the film was scored all the waythrough the hard coating layer and coating layer of the laminatedpolyester film down to the substrate film, producing 100 squares.Cellophane adhesive tape (No. 405, made by Nichiban; 24 mm wide) wasstuck to the scored squares and then rubbed with an eraser until itadhered completely. After this, the cellophane adhesive tape was peeledaway from the film perpendicularly, and the number of squares peeled offfrom the film was counted. The adhesion was determined from thefollowing equation. Any partially peeled squares were counted as havingpeeled.

Adhesion (%)=(1−number of peeled squares/100)×100

(2) Water Resistance

The polyester film produced by the method given in (1) above, which hada hard coating layer with a thickness of 3 μm provided on the coatinglayer, was immersed in warm water at 60° C. for 168 hours. The film wasthen taken out of the water, and the water clinging to the film surfacewas removed, and the film was left at room temperature for 12 hours.

The adhesion was then calculated by the same method as in (1) above, andwas ranked on the following scale.

⊚: 100%

◯: 99 to 96%

Δ: 95 to 80%

X: 79 to 0%

(3) Water Resistance Value

An offset ink (Best Cure 161, made by T & K Toka) was transferred withan RI tester (RI-3, made by Akira Seisakusho) onto the coating layersurface of the film. Then, a high pressure mercury vapor lamp was usedto form an ink layer with a thickness of 1 μm at an irradiation energyof 200 mJ/cm² and an irradiation distance of 15 cm while the film wasfed at a rate of 5 m/minute.

The film thus obtained was placed in an autoclave (SR-240, made by TomySeiko) containing water, and a boiling treatment under pressure wasperformed for 1 hour at 120° C. After this boiling treatment, thepressure in the autoclave was reduced to normal pressure and the filmwas taken out of the autoclave. Any water clinging to the film surfacewas removed, and the film was left to stand at room temperature for 12hours.

The surface on the ink layer side of the film after the boilingtreatment was then subjected to a peel test in the same manner as in (1)above, and the water resistance value was calculated from the followingequation. Any partially peeled squares were counted as having peeled.

Water resistance value (%)=(1−number of peeled squares/100)×100

(4) Color Tone

The Lab values were measured with a color meter (Z-1001DP, made byNippon Denshoku), and the b value was used.

(5) Discoloration Value After Melt Molding

The laminated polyester film having a coating layer was cut into strips,dried for 6 hours at 135° C. at a reduced pressure of 133 Pa, and thenfed to an extruder (PCM-30, made by Iketomo Tekkosha). The molten resinwas extruded in the form of a strand from a nozzle with a diameter of 5mm at a discharge rate of 200 g/minute, a cylinder temperature of 280°C., and a rotating speed of 80 rpm, and then the strand was cooled in awater tank and cut to obtain recovered pellets. It took 130 seconds fromthe start of the feeding of the film strips to the extruder until themolten resin began to flow out of the nozzle.

The b value of these recovered pellets (b) and the b value of thelaminated film prior to the test (b₀) were measured by a color meter,and the difference between these values was defined as discolorationvalue after melt molding.

Discoloration=b−b ₀

(6) Foreign Matter in Film

PET resin pellets (used as the film raw material polymer) that had anintrinsic viscosity of 0.62 dl/g and contained substantially noparticles were mixed with the recovered pellets produced in (5) above ina weight ratio of 60:40, and this pellet mixture was dried for 6 hoursat 135° C. at a reduced pressure of 133 Pa.

After drying, the pellet mixture was fed to an extruder (PCM-30, made byIketomo Tekkosha), melted and extruded from a T-die in the form of asheet at a cylinder temperature of 280° C., a discharge rate of 250g/minute, and a rotating speed of 150 rpm, and quickly cooled andsolidified on a metal roll whose surface temperature was maintained at20° C., by electropinning to prepare an unstretched sheet having athickness of 1400 μm. It took 310 seconds from the start of the supplyof the film strips to the extruder until the molten resin began to flowout from the T-die.

Then, this unstretched sheet was heated to 100° C. with an infraredheater and a group of heated rolls, and stretched to 3.5 times in thelongitudinal direction by using a group of rolls having differences inperipheral velocity to prepare a monoaxially oriented PET film. Then,the ends of the film were clamped with clips, and the film was guided toa hot-air zone heated at 130° C. where it was dried. Subsequently, thedried film was stretched to 4.0 times in the transverse direction,giving a biaxially oriented PET film with a thickness of 100 μm.

The film thus obtained was cut into a piece measuring 250 mm×250 mm andexamined under a scaled microscope, and the number of foreign particleshaving a diameter of at least 20 μm (when observed perpendicular to thefilm surface) was counted for the entire 250 mm×250 mm (0.0625 m²)range. This counting was performed for 10 film pieces, and the totalnumber of foreign particles thus counted was divided by the totalobserved area (0.625 m²) to calculate the number of foreign particlesper unit area 1 m² (number/m²), and this number was rounded off to thenearest whole number. The number of foreign particles per square meterwas ranked on the following scale.

⊚: 0 per m²

◯: 1 to 3 per m²

Δ: 4 to 6 per m²

X: 7 or more per m²

(7) Film Appearance

The film obtained above was observed using transmitted light andreflected light, and the condition of the film was observed visually andranked on the following scale. This observation was carried out by fivespecialists trained in such evaluation, and the most common ranking wasused as the evaluation ranking. If two rankings received the same numberof votes, the middle of the three different rankings was used. Forinstance, if “⊚” and “◯” each received two votes, and “Δ” one vote, aranking of “◯” was given; if “⊚” received one vote and “◯” and “Δ” eachreceived two votes, a ranking of “◯” was given; and if “⊚” and “Δ” eachreceived two votes, and “⊚” one vote, a ranking of “⊚” was given.

⊚: no discoloration, transparent and uniform

⊚: slight discoloration, but transparent and uniform

Δ: discoloration, with some cloudiness noted

X: marked discoloration, with cloudy or opaque portions noted

(8) Haze Value Change After Heating

The film was cut into two strips measuring 8 cm×10 cm, and measurementswere made twice at 8 points on each with a haze meter (TC-H3DP, made byTokyo Denshoku). The average of the measurement values at the 16 pointswas termed the initial haze value H₀ (%). These film strips were held byclips and heated for 30 minutes in a 150° C. hot air oven. The filmswere allowed to cool naturally, and the haze value after heating H₁ (%)was measured in the same manner as the above-mentioned initial hazevalue H₀. The difference between these haze values (H₁−H₀) is defined asthe haze value change after heating.

Preparation of Copolymer Polyester Resin

Into a stainless steel autoclave equipped with a stirrer, a thermometerand a partial reflux condenser were placed 163 weight parts of dimethylterephthalate, 163 weight parts of dimethyl isophthalate, 169 weightparts of 1,4-butanediol, 324 weight parts of ethylene glycol, and 0.5weight part of tetra-n-butyl titanate. A transesterification reactionwas conducted over a period of 4 hours while the temperature was raisedfrom 160° C. to 220° C.

Then, 14 weight parts of fumaric acid and 203 weight parts of sebacicacid were added, and an esterification reaction was conducted while thetemperature was raised from 200° C. to 220° C. over a period of 1 hour.The temperature was then raised to 255° C. and the pressure of reactionsystem was gradually reduced, and then the reaction was continued for 1hour and 30 minutes at a reduced pressure of 29 Pa, giving a copolymerpolyester resin (A-1). The copolymer polyester resin thus obtained wastransparent and pale yellow in color.

Copolymer polyester resins of other compositions (A-2 and A-3) wereobtained by the same method. Table 1 shows the composition as measuredby NMR and weight average molecular weight for A-1, A-2, and A-3.

TABLE 1 Copolymer composition (mol %) A-1 A-2 A-3 terephthalic acid   33  46   48 sebacic acid   30 — — isophthalic acid   33   46   48 sodium5-sulfonatoisophthalic —   4   4 acid fumaric acid   4   4 — ethyleneglycol   60   60   60 1,4-butanediol   40   40   40 Weight averagemolecular weight 18000 15000 19000 Aromatic component (mol %)   66   96 100

Example 1

(1) Manufacture of Graft Resin

To a reactor equipped with a stirrer, a thermometer, a reflux condenserand a metered dropping apparatus were added 75 weight parts of copolymerpolyester resin (A-1), 56 weight parts of methyl ethyl ketone, and 19weight parts of isopropyl alcohol. The contents were heated to 65° C.and stirred, and the resin was dissolved. Once the resin was completelydissolved, 15 weight parts of maleic anhydride were added to thepolyester solution.

10 weight parts of styrene and 1.5 weight parts ofazobisdimethylvaleronitrile were dissolved in 12 weight parts of methylethyl ketone, the resulting solution was added dropwise to a polyestersolution at a rate of 0.1 mL/min, and continuously stirred for another 2hours. An analysis sample was taken from the reaction solution, and then5 weight parts of methanol was added. To the reaction solution were thenadded 300 weight parts of water and 15 weight parts of triethylamine,and the system was stirred for 1 hour.

The temperature inside the reactor was then raised to 100° C. and themethyl ethyl ketone, isopropyl alcohol, and excess triethylamine weredistilled off, giving a water-dispersible graft copolymer resin B-1.This water-dispersible graft resin B-1 was transparent and pale yellowin color. The acid value of this graft copolymer was 1400 eq/t.

(2) Preparation of Coating Solution

40 weight parts of a 25 wt % aqueous dispersion of the water-dispersiblegraft resin B-1 obtained above, 24 weight parts of water, and 36 weightparts of isopropyl alcohol were mixed, and to the mixture were addedpropionic acid and an anionic surfactant in an amount of 1 wt % eachwith respect to the coating solution, and an aqueous dispersion ofcolloidal silica fines (Snowtex OL, made by Nissan Chemical Industries;average particle diameter: 40 nm) in an amount of 5 wt % silica withrespect to the resin solids, thereby preparing a coating solution(hereinafter referred to as coating solution C-1).

(3) Manufacture of a Laminated Polyester Film Having a Coating Layer

PET resin pellets (used as the film raw material polymer) that had anintrinsic viscosity of 0.62 dL/g and contained substantially noparticles were dried for 6 hours at 135° C. at a reduced pressure of 133Pa. They were then supplied to a biaxial-extruder, melted and extrudedin the form of a sheet-at approximately 280° C., and quickly cooled andsolidified on a rotating matal roll whose surface temperature wasmaintained at 20° C., and adhered to each other by electropinning toprepare an unstretched PET sheet having a thickness of 1400 μm.

This unstretched sheet was heated to 100° C. by a group of heated rollsand an infrared heater, and stretched to 3.5 times in the longitudinaldirection by using a group of rolls having differences of peripheralvelocity to prepare a monoaxially oriented PET film.

Both sides of the PET film were then coated by the reverse roll methodwith the above-mentioned coating solution C-1 such that the coatingamount after drying would be 0.6 g/m², and the coatings were dried for20 seconds at 80° C. After drying, the film was stretched to 4.0 timesin the transverse direction by a tenter at 120° C., the coating layerwas heated for 0.5 second at 260° C. with an infrared heater with thelength fixed in the transverse direction of the film, then a relaxationtreatment of 3% in the transverse direction was performed for 23 secondsat 200° C., giving a biaxially oriented PET film with a thickness of 100μm. The evaluation results are given in Table 2.

Example 2

A coating solution C-2 was obtained by the same method as above, exceptthat the 15 weight parts of maleic anhydride and 10 weight parts ofstyrene were changed to 10 weight parts of maleic anhydride, 7 weightparts of styrene, and 8 weight parts of ethyl acrylate. The acid valueof this graft copolymer was 950 eq/t. This coating solution was used toobtain a laminated biaxially oriented PET film by the same method as inExample 1. The evaluation results are given in Table 2.

Example 3

A coating solution C-3 was obtained by the same method as in Example 1,except that the copolymer polyester resin was changed to A-2. The acidvalue of this graft copolymer was 1370 eq/t. This coating solution wasused to obtain a laminated biaxially oriented PET film by the samemethod as in Example 1. The evaluation results are given in Table 2.

Example 4

A laminated biaxially oriented PET film was obtained by the same methodas in Example 1, except that the thickness of the unstretched PET sheetwas changed to 2632 μm, the thickness of the PET film after biaxialstretching was changed to 188 μm, the drying conditions after coatingwere changed to 40 seconds at 70° C., thermal fixing was performed for0.6 second at 260° C., and a relaxation treatment of 3% was performedfor 47 seconds at 200° C. The evaluation results are given in Table 2.

Example 5

A laminated biaxially oriented PET film was obtained by the same methodas in Example 1, except that 1 weight part of a self-crosslinkingpolyurethane resin having blocked isocyanate groups (Elastron H-3, madeby Dai-ichi Kogyo Seiyaku) and 0.1 weight part of an Elastron catalyst(Cat64, made by Dai-ichi Kogyo Seiyaku) were added per 100 weight partsof the coating solution C-3. The evaluation results are given in Table2.

Example 6

A laminated biaxially oriented PET film was obtained by the same methodas in Example 1, except that the coating solution C-1 used in Example 1was applied by the reverse roll method, to only one side of the PETfilm. The evaluation results are given in Table 2.

Comparative Example 1

A laminated biaxially oriented PET film was obtained by the same methodas in Example 1, except that only an aqueous dispersion of the copolymerpolyester resin A-3 was used as a coating solution C-4. The evaluationresults are given in Table 2.

Comparative Example 2

A laminated biaxially oriented PET film was obtained by the same methodas in Example 1, except that 10 weight parts of a self-crosslinkingpolyurethane resin having blocked isocyanate groups (Elastron H-3, madeby Dai-ichi Kogyo Seiyaku) and 1 weight part of an Elastron catalyst(Cat64, made by Dai-ichi Kogyo Seiyaku) were added per 100 weight partsof the coating solution C-4. The evaluation results are given in Table2.

Comparative Example 3

A laminated biaxially oriented PET film was obtained by the same methodas in Example 1, except that 10 weight parts of melamine resin (SumimalM40W, made by Sumitomo Chemical) and 0.02 weight part p-toluenesulfonicacid were added per 100 weight parts of the coating solution C-4. Theevaluation results are given in Table 2.

Comparative Example 4

A laminated biaxially oriented PET film was obtained by the same methodas in Example 1, except that 20 weight parts melamine resin (SumimalM40W, made by Sumitomo Chemical) and 0.05 weight part p-toluenesulfonicacid were added per 100 weight parts of the coating solution C-4. Theevaluation results are given in Table 2.

Comparative Example 5

A laminated biaxially oriented PET film was obtained by the same methodas in Comparative Example 1, except that an acrylic copolymer resin A-4synthesized from methyl methacrylate, ethyl acrylate, and 2-hydroxyethylacrylate was used instead of the copolymer polyester resin A-3. The acidvalue of this copolymer was 2 eq/t. The evaluation results are given inTable 2.

Comparative Example 6

A laminated biaxially oriented PET film was obtained by the same methodas in Example 1, except that no propionic acid was added to the coatingsolution. The evaluation results are given in Table 2.

Comparative Example 7

A laminated biaxially oriented PET film was obtained by the same methodas in Example 1, except that the coating layer was not heated with aninfrared heater. The evaluation results are given in Table 2.

INDUSTRIAL APPLICABILITY

The present invention provides a laminated polyester film with excellentadhesion and water resistance, and therefore can be used in applicationsinvolving a wide range of materials, such as photographic photosensitivelayers, diazo photosensitive layers, mat layers, magnetic layers, inklayers, adhesive agent layers, thermosetting resin layers, UV-curedresin layers, and vapor deposited layers of metal or inorganic oxides.Because there is little haze value change after heating, this film isfavorable as a base film for optical components and in printingapplications that involve heat treatment in an after-processing step.Furthermore, since any scrap film which does not become a finishedproduct in the film manufacture can be recovered and reused as a filmraw material, the present invention is also useful from the standpointsof cost and environmental protection.

TABLE 2 Film from Water Discoloration Change in haze recovered pelletsresistance after melt value after Water Foreign Adhesion value moldingheating resistance matter Appearance Ex. 1 100 100 3.7 8.6 ⊚ ∘ ∘ Ex. 2100 100 3.9 9.3 ⊚ ∘ ∘ Ex. 3 100 98 4.6 7.5 ∘ ∘ ∘ Ex. 4 100 100 2.9 8.0 ⊚∘ ∘ Ex. 5 100 100 6.8 9.8 ⊚ ∘ ∘ Ex. 6 100 100 3.2 13.4 ⊚ ∘ ∘ Comp. Ex. 195 10 4.3 14.3 Δ ∘ Δ Comp. Ex. 2 100 60 12.9 23.1 x Δ x Comp. Ex. 3 10070 11.4 14.7 Δ x x Comp. Ex. 4 100 90 16.0 11.0 O x x Comp. Ex. 5 75 254.2 17.0 x ∘ Δ Comp. Ex. 6 80 85 3.5 24.0 Δ ∘ ∘ Comp. Ex. 7 80 70 3.622.6 Δ ∘ ∘

What is claimed is:
 1. A laminated polyester film having a coating layerformed by using an aqueous coating solution which comprises mainly anaqueous solvent and a water-soluble or water-dispersible resin, and issubstantially free from phenolic and nitrogenous crosslinking agents onat least one side of the polyester film, the water-soluble orwater-dispersible resin comprising one of (a)-(c) below: (a) a graftcopolymer of an aqueous aromatic polyester resin containing at least 5%by weight of a radical polymer at least one of whose monomers comprisesan acid anhydride having a double bond; (b) an aqueous acrylic resin,other than (c) below, having an acid value of at least 200 eq/t, or acopolymer of such a resin; or (c) a graft copolymer of an aqueousacrylic resin containing at least 5% by weight of a radical polymer atleast one of whose monomers comprises an acid anhydride having a doublebond, the acid value of the aqueous acrylic resin being at least 200eq/t; wherein the laminated film has a water resistance value of atleast 90%, as determined by forming an ink layer on the coating layerside of the laminated film, performing a boiling treatment underpressure for an hour at 120° C., conducting a peel test according to JISK 5400, and measuring the area (%) of the remained ink that is notpeeled off; the difference between the b value of recovered pelletsobtained by melt extruding the laminated film at 280° C. and the b valueof the laminated film before melting (discoloration value after meltmolding) is not more than 10; and the difference between the haze valueof the laminated film after heating for 30 minutes at 150° C. and thehaze value of the laminated film before heating (change in haze valueafter heating) is not more than 20%.
 2. The laminated polyester filmaccording to claim 1, wherein the coating solution further comprises acrosslinking agent free from phenolic or nitrogenous components.
 3. Thelaminated polyester film according to claim 1, wherein the coatingsolution further comprises an acid compound.
 4. The laminated polyesterfilm according to claim 1, wherein the coating weight of the coatinglayer after drying is 0.01 to 1.0 g/m².
 5. The laminated polyester filmaccording to claim 1, wherein the film is used for printingapplications.
 6. The laminated polyester film according to claim 1,wherein the film is used as a substrate film for an optical component.7. A method for producing a laminated polyester film comprising thesteps of: applying an aqueous coating solution which comprises mainly anaqueous solvent and a water-soluble or water-dispersible resin, and issubstantially free from phenolic and nitrogenous crosslinking agents onat least one side of an unstretched polyester film on at least one sideof a uniaxially oriented polyester film to form a coating layer; dryingthe film; orienting the film at least in one axial direction; andperforming thermal fixing and crosslinking of the resin; thewater-soluble or water-dispersible resin comprising one of (a)-(c)below: (a) a graft copolymer of an aqueous aromatic polyester resincontaining at least 5% by weight of a radical polymer at least one ofwhose monomers comprises an acid anhydride having a double bond; (b) anaqueous acrylic resin, excluding (c) below, having an acid value of atleast 200 eq/t, or a copolymer of such a resin; or (c) a graft copolymerof an aqueous acrylic resin containing at least 5% by weight of aradical polymer at least one of whose monomers comprises an acidanhydride having a double bond, the acid value of the aqueous acrylicresin being at least 200 eq/t; wherein the crosslinking step isperformed by (i) adding an acid compound to the coating solution, (ii)heating the coating layer with an infrared heater in a manner such thatthe length of the film is fixed in the transverse direction, or (iii)both.
 8. The method for producing a laminated polyester film accordingto claim 7, wherein the coating solution further comprises acrosslinking agent free from phenolic and nitrogenous components.